The present invention relates to therapeutic ultrasonic transducers and particularly to arrangements for controlling the effective beam width of therapeutic ultrasonic radiation and avoiding cavitation and other effects arising out of excess energy in a portion of the tissue being treated. In particular, the invention has application in treatment of ocular diseases, including glaucoma, retinal detachment and tumors.
It has been previously known to provide therapeutic ultrasonic radiation by use of a spherical cap transducer. This transducer provides a symmetrical focused beam with a circular cross-section at the focal plane. Such beams can provide thermally induced lesions with circular cross-sections, but are not optimum for treatment of tumors because the narrow circular beam requires a large number of exposures to treat the entire tumor region.
Another problem associated with a narrow circular beam is that production of broader therapeutic lesions can require excess energy intensity at or near the focal point, which can lead to cavitation, vaporization or tissue degassing, creating gas bodies that block ultrasound transmission to deeper tissue regions.
A prior approach has been described which uses annular transducer segments to control and adjust focusing of the transducer beam. This approach does not effectively spread the energy in one dimension only and can lead to off focal plane "hot" spots that can cause cavitation etc.
Another prior approach has been to use a planar phased array of transducer elements. Still another approach uses annular sectors on a spherical shell transducer. These approaches are complex and require provision of complex radio frequency (RF) amplitude and phase control circuits.
It is an object of the invention to provide an improved therapeutic ultrasound transducer with a beam having a broad pattern in one direction and a narrow pattern in another transverse direction to provide easier and more effective therapeutic treatment with high intensity focused ultrasound.
It is a further object to provide a transducer that avoids high intensity regions that can cause cavitation, vaporization or tissue degassing effects.